In today's Gospel (Mk 1:40-45 ), we hear Jesus referring back to the Old Testament reading (Lv 13:1-2, 44-46) and Levitical law. First, if you aren't aware of it, this is a great example of the structure and planning that's gone into the Mass readings. If you consider them together, you'll often see them interacting, reemphasizing a point or visiting it from another angle. The connections between documents thousands of years apart in human authorship amazes me every time I consider it. There can be no question that the Scriptures are inspired.
Additionally, the reading from Leviticus shows us God passing on to Moses what is essentially medical advice. This isn't immutable moral law like the Ten Commandments; this is good advice for a community that's try to stay healthy and survive. Only recently (relatively speaking) have we become aware of how disease is transmitted. God already knew, of course, and He passed just enough information on to Moses and His people to keep them going.
Doesn't the same happen in our lives? There's just enough information. God gives us a candle, not a spotlight. He picks us up on our feet, rather than carry us along. God knows far more than we do, and He guides us with that knowledge even if we don't always understand the "why".